Before you use either Windows PowerShell or Hyper-V Manager to shrink a VHD or VHDX file, you should log on to the guest operating system and use the Disk Management snap-in to shrink the volume on the virtual disk you want to shrink. For more information, see Ben Armstrong’s post titled “Shrinking a virtual hard disk in Windows 8” in his “Virtual PC Guy Blog” at http://blogs.msdn.com/b/virtual_pc_guy/archive/2012/05/07/shrinking-a-virtual-hard-disk-in-windows-8.aspx.
The above tip was excerpted from Mitch Tulloch's book Training Guide: Installing and Configuring Windows Server 2012 from Microsoft Press.
Mitch is a nine-time recipient of the Microsoft Most Valuable Professional (MVP) award and a widely recognized expert on Windows administration, deployment and virtualization. For more information see http://www.mtit.com.